It is known to encapsulate water insoluble perfumes or other non-fragrance materials, in small capsules often termed microcapsules, typically having diameters between 1 and 1000 micrometers (microns μm), for a variety of reasons relating to the protection, delivery and release of the perfume or other beneficial materials from consumer products.
Microcapsules are described in Kirk Othmer's Encyclopaedia of Chemical Technology 5th edition. One type of microcapsules, referred to as a wall or shell or core shell microcapsule, comprises a generally spherical shell of water and oil insoluble materials, typically a network polymer material, within which perfume or other hydrophobic material is contained.
Microcapsules can be prepared by many chemical reaction types and differing reagents. Many of the reactions used to prepare polymers can be adapted to prepare microcapsule walls. Amongst the most common chemical reactions used to prepare microcapsules having particle sizes greater than 1 μm are: condensation reactions such as those involving aldehydes such as formaldehyde or glutaraldehyde and amine or phenolic compounds such as melamine, urea or resorcinol. Microcapsules of this type are currently used in household products.
Co-acervation is another encapsulation technique which is widely used to form microcapsules in which pre formed polymers react together such that the product is insoluble in both aqueous and oil phases and forms a protective shell at the interface of an oil and water emulsion. Interfacial polymerization is a further option and involves the reaction of oil soluble compounds with water soluble compounds to form a network of polymers which are insoluble in both oil and water. Examples of such reactions are oil soluble acid chlorides such as diterephthaloyl chloride reacting with water soluble amines to form shell walls. Free radical polymerization of unsaturated compounds such as vinylic, acrylic, or styrenic compounds is another way of producing microcapsules and is becoming increasingly important as manufacturers either try to avoid compounds regarded as unsafe in some way, or to solve the problems encountered with other microcapsule types. To those skilled in the art it is recognized that it is possible to combine these various microcapsule wall forming processes so as to either form sequential layers of microcapsule walls or a single wall using combinations of reactions or post capsule formation to add additional materials to modify the properties of the wall.
Current microcapsules are not entirely satisfactory. The manufacturing process may leave residual amounts of chemicals such as formaldehyde in the product which is undesirable. It is also known that microcapsule contents may leak during storage in the product. For these and other reasons manufacturers still seek improved methods of microcapsule production. The current invention addresses another specific problem of microcapsules in liquid products, that of maintaining a homogeneous microcapsule dispersion in a liquid product. Depending on the size of the microcapsules, viscosity of the liquid phase and density difference between the liquid phase and microcapsules this problem may be more or less serious. Microcapsules containing fragrance oils, made by any process in which the starting materials used to prepare the microcapsule wall are predominantly of low density or result in a thin walled capsule may separate on storage due to density differences and it can be difficult to keep such microcapsules uniformly dispersed in aqueous or higher density, liquid products.
When such microcapsules are incorporated in liquid consumer products e.g. personal care products such as shampoos, hair conditioners, body washes, shower gels, laundry products such as fabric conditioners or liquid laundry detergents or household cleaners such as kitchen surface cleaners, problems can arise, with the microcapsules either creaming (rising to the surface) or settling over time, especially while the product is stored. The creaming or settling is due to differences in density between the microcapsule and the surrounding liquid. Many aqueous based consumer products liquid household cleaners, liquid laundry products and personal care and cosmetic products have densities around 1.00 gram per cubic centimeter (g/cm3), while many organic compounds have densities much lower than 1.00 g/cm3. So a microcapsule containing a high proportion of fragrance oils or other hydrophobic oils may have a lower density than the liquid phase of the product in which the microcapsules are dispersed, hence these microcapsules will tend to rise or cream over time. If the microcapsule wall material is thin, or made from lower density starting materials this creaming phenomenon will be more noticeable.
Since it may not be desirable or even possible to prepare microcapsules of different (usually smaller) size to reduce creaming as this may have other consequences, such as affecting the ease of breaking the walls for those microcapsules which rely on friability for content release. Moreover less material is encapsulated into a smaller microcapsule requiring a higher proportion of wall material relative to content and a larger number of microcapsules to contain the same volume of core material which consequently may affect product attributes such as colour and also the manufacturing cost. It may also be undesirable to increase the viscosity of the liquid product in which the microcapsules are dispersed, hence it is advantageous if the densities of the microcapsules and liquid phase can be more equally balanced.
Patent Literature 1 relates to core shell microcapsules in which high density cyclic fragrance materials are encapsulated. The shell of the microcapsules mainly comprises an aminoplast resin.
Patent Literature 2 describes adding solvents to core shell encapsulated fragrances but stipulates that the fragrances must have ClogP greater than 3.3, preferably greater than 8.
Patent Literature 3 describes density modifiers for co-acervate microcapsules for detergent liquid products but only describes materials which lower the density of the microcapsule.
Patent Literature 4 also describes modifying the microcapsule contents in order to balance the density with the surrounding liquid.
Patent Literature 5 describes balancing the densities of poly alpha olefin particles with surface coatings of wax and relatively high density inorganic or mineral particulates.
Patent Literature 6 describes modifying the density of fragrance containing microcapsules using high density materials in the core. Particularly exemplified are inorganic materials such as titanium dioxide although other high density materials such as brominated vegetable oil are also mentioned.